The term "tabular grain" is employed to indicate a silver halide grain having an aspect ratio of at least 2, where "aspect ratio" is ECD/t, ECD being the equivalent circular diameter of the grain (the diameter of a circle having the same projected area as the grain) and t is the thickness of the grain.
The term "ultrathin tabular grain" is employed to indicate a tabular grain of a thickness less than 0.07 .mu.m.
The term "tabular grain emulsion" is employed to indicate an emulsion in which tabular grains account for at least 50 percent of total grain projected area.
The term "high chloride" or "high bromide" as applied to a grain or emulsion is employed to indicate that the grain or the grains of the emulsions contain at least 50 mole percent chloride or bromide, respectively, based on total silver present in the grain or the grains of the emulsion.
The term "{111} tabular grain" is employed to indicate an emulsion in which the parallel major faces of the tabular grain lie in {111} crystal planes.
The first high chloride high aspect ratio (ECD/t&gt;8){111} tabular grain emulsion is disclosed in Wey U.S. Pat. No. 4,399,215. The grains were relatively thick. Maskasky U.S. Pat. No. 4,400,463 (hereinafter designated Maskasky I) obtained thinner high chloride {111} tabular grains by employing an aminoazaindene (e.g., adenine) in combination with a synthetic peptizer having a thioether linkage. Maskasky U.S. Pat. No. 4,713,323 (hereinafter designated Maskasky II) produced thinner high chloride {111} tabular grains by employing the aminoazaindene grain growth modifier in combination with low methionine (&lt;30 micromole per gram) gelatin, also referred to as "oxidized" gelatin, since the methionine concentration is reduced by employing a strong oxidizing agent, such as hydrogen peroxide.
High chloride ultrathin {111} tabular grain emulsions are disclosed in Maskasky U.S. Pat. No. 5,217,858 (hereinafter designated Maskasky III). Maskasky III discloses to be effective in preparing high chloride ultrathin {111} tabular grain emulsions triaminopyrimidine grain growth modifiers containing 4, 5 and 6 ring position amino substituents, with the 4 and 6 position substituents being hydroamino substituents. The term "hydroamino" designates an amino group containing at least one hydrogen substituent--i.e., a primary or secondary amino group. The triaminopyrimidine grain growth modifiers of Maskasky III include both those in which the three amino groups are independent (e.g., 4,5,6-triaminopyrimidine) and those in which the 5 position amino group shares a substituent with 4 or 6 position amino group to produce a bicyclic compound (e.g., adenine, 8-azaadenine, or 4-amino-7,8- dihydro-pteridine).
The process which Maskasky III employs to prepare high chloride ultrathin {111} tabular grain emulsions is a double jet process in which silver and chloride ions are concurrently run into a dispersing medium containing the grain growth modifier. The first function of the grain growth modifier is to promote twinning while grain nucleation is occurring, so that ultrathin grains can form. Thereafter the same grain growth modifier or another conventional grain growth modifier can be used to stabilize the {111} major faces of the high chloride tabular grains.
A common feature of the Maskasky high chloride {111} tabular grain emulsion precipitations is the presence of a grain growth modifier. The reason for this is that high chloride {111} tabular grains, unlike high bromide {111} tabular grains, cannot be formed or maintained in the absence of a grain growth modifier, but rather take nontabular forms, since {100} crystal faces are more stable in high chloride grains.
The art has long recognized that distinctly different techniques are required for preparing high chloride {111} tabular grain emulsions and high bromide {111} tabular grain emulsions. For example, Maskasky III does not disclose the processes of preparing high chloride ultrathin {111} tabular grain emulsions to be applicable to the preparation of high bromide ultrathin {111} tabular grain emulsions. Further, since at low pBr the {111} major faces of high bromide tabular grains have no tendency to revert to {100} crystal faces, the precipitation of high bromide {111} tabular grain emulsions has not required the addition of compounds comparable to the grain growth modifiers of Maskasky.
Daubendiek et al U.S. Pat. 4,914,014, Antoniades et al U.S. Pat. 5,250,403 and Zola et al EPO 0 362 699 illustrate the preparation of high bromide ultrathin {111} tabular grain emulsions. Each of the Examples resulting in the formation of ultrathin tabular grain emulsions are replete with adjustments undertaken during precipitation. Typical complexities include (a) different pBr conditions for grain nucleation and growth, (b) interruptions of the silver and/or halide salt additions, (c) frequent modifications of the rate of silver and/or halide salt additions, (d) the use of separate reaction vessels for grain nucleation and growth, thereby at least doubling the complexity of reaction vessel and control equipment, (e) the variance in dispersing medium volume as precipitation progresses, which makes optimized reaction vessel sizing for all phases of precipitation impossible, (f) dilution of emulsion silver content as precipitation progresses toward completion, thereby creating a water removal burden and increasing the required capacity of the reaction vessel, and (g) when pBr is maintained at customary low (e.g., pBr&lt;1.5) values employed for precipitating high bromide {111} tabular grain emulsions, large excess amounts of soluble bromide salts must be discarded. Note that since pBr is the negative logarithm of bromide ion activity, bromide ion concentrations increase as pBr decreases. This is directly analogous to hydrogen ion activity increasing as pH decreases. None of Antoniades, Daubendiek et al and Zola et al suggest the use of any compound comparable to a grain growth modifier to prepare high bromide ultrathin {111} tabular grain emulsions.
Verbeeck EPO 0 503 700 discloses reduction of the coefficient of variation (COV) of high bromide high aspect ratio {111} tabular grain emulsions through the presence of an aminoazaindene, such as adenine, 4-aminopyrazolopyrimidine and substitutional derivatives, prior to the precipitation of 50 percent of the silver. Double jet precipitation techniques are employed. The minimum disclosed thickness of a tabular grain population is 0.15 .mu.m.